1. Field of the Invention
The present invention relates to a tunable chirped fiber grating device in which the spacings of perturbations in the index of refraction vary with a strain which is produced by applying a voltage across a predetermined piezoelectric element and a method for forming a chirped fiber grating.
2. Description of the Related Art
Typically, a chirped grating is designed to have different perturbation spacings or different effective refractive indices depending on positions of the perturbations, and is characterized in that the wavelength of reflected light differs depending on the positions of the perturbations. FIGS. 1A and 1B show side views illustrating an optical fiber in which a chirped grating is formed. Here, a core is represented by reference numeral 100 or 120, a cladding by 110 or 130, and a grating by 101 or 102. The chirped grating 101 in the core 100 according to FIG. 1A has a structure in which the spacings of successive perturbations are different from each other, while the effective refractive indices of the perturbations are the same, and a chirped grating 102 in the core 120 according to FIG. 1B has a structure in which the spacings of successive perturbations are the same, while the effective refractive indices of the perturbations are different from each other.
However, chirped gratings in which the spacings of successive perturbations are different are doomed to experience more complicated procedures than those having equal perturbation spacings. Since the chirping rates of the reflected wavelengths according to the positions of the perturbations are fixed to values in fabrication conditions and the gratings are used for a specific wavelength band, the gratings cannot be flexibly applied (manufactured).
Forming chirped gratings is known in the art as exemplified by U.S. Pat. No. 5,781,670 to is David A. G. Deacon, et al. entitled Optical Frequency Channel Selection Filter With Electronically-controlled Grating Structures; U.S. Pat. No. 5,718,738 to Glenn Eric Kohnke et al. entitled Method For Making Continuously Chirped Fiber Bragg Gratings; U.S. Patent No. 5,636,304 to Victor Mizrahi et al. entitled Article Comprising A Spa tially Varying Bragg Grating In An Optical Fiber; and U.S. Pat. No. 5,007,705 to William W. Morey et al. entitled Variable Optical Fiber Bragg Filter Arrangement.